I also post "Thank you" posts to everyone that subscribes to my comic, however, I do try to write a personal thank you to each subscriber. Insert a little joke, comment on their avatar, bio, user name, or look at their reading list and comment on it, maybe suggest another comic they might love. It's been a great way to genuinely thank each reader for following, and awesome friendships and conversations have started from that.
Also, I've found a lot of unknown great comics by looking at other reader's wall posts and reading list. So I don't see them as a bad thing, even if they are a bit of a shameless plug. However, none of them actually link back to their comics (that I've seen), in my eyes, that makes it less spammy. They're not littering my wall with links but with beautiful artwork (is that really a bad thing?).
idk, as long as intentions are good, I don't have a problem with it. And as a reader, I'm happy when a comic I like and follow posts to my wall. I hope other people will find it too, and if I can help them achieve that, awesome.